One form of procedure for the production of tubular bodies for packaging tubes involves producing the tubular bodies from a foil strip which includes a weldable plastic material, by virtue of the longitudinal edges of the foil strip being thermally joined together in mutually overlapping relationship. The thermal joining or seaming operation is carried out in an apparatus referred to as a longitudinal seam welding machine or side seamer, which may comprise a shaping belt for shaping the foil strip around a mandrel to provide the tubular body, with a lower driven transportation belt running against the mandrel and an upper driven transportation belt, with the longitudinal edges of the foil strip being received between the belts in mutually overlapping relationship. A heating means acts on the upper transportation belt to cause the overlapping longitudinal edges to fuse together and the fused edges are then pressed, with pre-hardening of the plastic material. The pre-hardened plastic material is then finally hardened under the action of a cooling means disposed downstream of the pressing means. The upper and lower transportation belts which thus run one above the other are generally made of metal so that the action of heat and pressure on the belts causes the material of the overlapping edges to be fused together in a fused seam before the resulting seam is then cooled to effect hardening thereof.
In such an apparatus, the upper transportation belt passes around sets of rollers, wherein the heat generating, pressing and cooling means are disposed between the upper and lower runs of the upper belt, being adapted to act on the lower run thereof. In the part of the apparatus in which the fusing, pressing and cooling operations, that is to say the actual operation of forming the welded seam, takes place, the lower transportation belt runs in the same direction as the upper belt, in the mandrel referred to above.
In that arrangement, the upper run of the lower belt is guided on a surface of the mandrel and co-operates with the lower run of the upper belt in the manner of a support means while the lower run of the lower belt experiences a change in its direction of movement of the mandrel and goes back in the mandrel.
The upper run of the lower belt may be heated before it passes on to the mandrel, by means of external heating devices which apply heat to the upper run of the belt. The mandrel may also include heating inserts on which the upper run of the lower belt moves. In order to accelerate cooling of the welded seam, the mandrel may be cooled in the section which co-operates with the cooling means that acts on the upper transportation belt.
The mandrel in the above-indicated apparatus is the cause of limitations in terms of an increase in operating speed and economy of longitudinal seam welding machines or side seamers, such limitations being due to the quality requirements in respect of the welded seams. It is not possible to make use of tubular bodies with defective welded seams, welding which is incomplete in places on the tubular body or seams which have unwelded material squeezed out of the edge portions of the mutually overlapping longitudinal edges of the foil strip, as they could result in the packaged material being unacceptably contaminated.
In an endeavor to eliminate those and other seam defects when using high operating speeds, the heat applied to and the heat removed from the weld, by way of the two belts, that is to say the lower run of the upper belt and the upper run of the lower belt which thus act as welding tools, in conjunction with the effect of pressure applied to the weld, are to be kept closely matched to the nature and material of the plastic foil and the speed of movement, that is to say the operating speed of a side seamer.
In regard to the upper belt, it is substantially possible to achieve good matching in relation to laminate materials by virtue of adjustability of the individual action of the heating means, pressing means and cooling means, which represent individual elements that are structurally separate from each other, and which thus constitute sections of the arrangement which are individualized in terms of heat produced and pressure applied as well as cooling effect, over the welding section of the apparatus.
However the mandrel in the above-indicated apparatus cannot provide for a comparable matching effect, for the reason that, when the belt is heated and cooled upstream of and in the mandrel, the belt experiences a temperature gradient which decreases in the direction of the cooling portion, and that temperature gradient can be adjusted overall, but it cannot be adjusted in an individualized fashion for each section, in order to provide a good matching effect. That means that the above-indicated mandrel which contributes to the welding process determines to a not inconsiderable degree the operating speed and economy of side seamers as well as the quality of the welded seams produced.